Transglutaminase 2 in neurodegenerative disorders

Type 2 transglutaminase (TG2) is a calcium-dependent acyltransferase which also undergoes a GTP-binding/GTPase cycle even though it lacks any obvious sequence similarity with canonical GTP-binding (G) proteins. As an enzyme which is responsible for the majority of transglutaminase (TG) activity in the brain, TG2 is likely to play a modulatory role in nervous system development and has regulatory effect on neuronal cell death as well. Most importantly, numerous studies have presented data demonstrating that dysregulation of TG2 may contribute to the pathogenesis of many neurodegenerative disorders, including Huntington's disease, Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis as well as nervous system injuries. Although TG2's involvement in these disease conditions is strongly suggested by various findings, such as the increase of TG2 mRNA expression, protein level and TG activity in the pathological process of these neurodegenerative disorders, as well as the therapeutic effect of TG2 genetic deletion in animal models of Huntington's disease, the precise mechanism underlying TG2's role remain unclear. TG2 was originally proposed to contribute to the pathogenesis of these diseases by facilitating the formation of insoluble protein aggregates, however recent findings clearly indicate that this is likely not the case. Nonetheless, there is data to suggest that TG2 may play a role in neurodegenerative processes by stabilizing toxic oligomers of the disease-relevant proteins, although further studies are needed to validate these initial in vitro findings.

Activation of Src-family kinases in spinal microglia contributes to mechanical hypersensitivity after nerve injury

Hypersensitivity to mechanical stimulation is a well documented symptom of neuropathic pain, for which there is currently no effective therapy. Src-family kinases (SFKs) are involved in proliferation and differentiation and in neuronal plasticity, including long-term potentiation, learning, and memory. Here we show that activation of SFKs induced in spinal cord microglia is crucial for mechanical hypersensitivity after peripheral nerve injury. Nerve injury induced a striking increase in SFK phosphorylation in the ipsilateral dorsal horn, and SFKs were activated in hyperactive microglia but not in neurons or astrocytes. Intrathecal administration of the Src-family tyrosine kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) suppressed nerve injury-induced mechanical hypersensitivity but not heat and cold hypersensitivity. Furthermore, PP2 reversed the activation of extracellular signal-regulated protein kinase (ERK), but not p38 mitogen-activated protein kinase, in spinal microglia. In contrast, there was no change in SFK phosphorylation in primary sensory neurons, and PP2 did not decrease the induction of transient receptor potential ion channel TRPV1 and TRPA1 in sensory neurons. Together, these results demonstrate that SFK activation in spinal microglia contributes to the development of mechanical hypersensitivity through the ERK pathway. Therefore, preventing the activation of the Src/ERK signaling cascade in microglia might provide a fruitful strategy for treating neuropathic pain.

Elevation of superoxide dismutase increases acoustic trauma from noise exposure

The generation of superoxide has been implicated as a cause of cochlear damage from excessive noise. Cu/Zn superoxide dismutase (SOD1) generally will protect against superoxide-mediated tissue injury but protection by this enzyme against noise trauma is controversial. This study assessed auditory function in C57BL/6 mice overexpressing SOD1 or treated with lecithinized SOD1 (PC-SOD1). Noise exposure caused significantly higher threshold shifts in PC-SOD1-treated animals than physiological saline-treated animals. Cochlear tissues of PC-SOD1-treated animals exhibited significant elevation of the levels in the SOD activity, not in the catalase activity, in comparison with those of saline-treated animals. Likewise, transgenic mice overexpressing SOD1 tended to suffer higher threshold shifts than nontransgenic littermates from noise exposure. The findings indicate that increasing SOD1 enhances auditory dysfunction following noise exposure.

Regulation of stearoyl-CoA desaturase-1 after central and peripheral nerve lesions

Background

Interruption of mature axons activates a cascade of events in neuronal cell bodies which leads to various outcomes from functional regeneration in the PNS to the failure of any significant regeneration in the CNS. One factor which seems to play an important role in the molecular programs after axotomy is the stearoyl Coenzyme A-desaturase-1 (SCD-1). This enzyme is needed for the conversion of stearate into oleate. Beside its role in membrane synthesis, oleate could act as a neurotrophic factor, involved in signal transduction pathways via activation of protein kinases C.

Results

In situ hybridization and immunohistochemistry demonstrated a strong up-regulation of SCD at mRNA and protein level in regenerating neurons of the rat facial nucleus whereas non-regenerating Clarke's and Red nucleus neurons did not show an induction of this gene.

Conclusion

This differential expression points to a functionally significant role for the SCD-1 in the process of regeneration.

Nitric oxide synthase/nicotinamide adenine dinucleotide phosphate-diaphorase in the brainstem trigeminal nuclei after transection of the masseteric nerve in rats

In this study, the responses of nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) and neuronal nitric oxide synthase (nNOS) activities were quantitatively analyzed at different times in both ipsilateral and contralateral sides of trigeminal nuclei, after unilateral trigeminal muscle nerve transection, in Sprague Dawley rats. In the control animals, both NADPH-d- and nNOS-positive neurons were constitutively distributed in the rostrolateral solitary tract nucleus, dorsomedial part of trigeminal nucleus oralis (Vo/Sn), and superficial layers (VcI/II) of the trigeminal nucleus caudalis (Vc). NADPH-d-positive neurons appeared in the trigeminal mesencephalic nucleus ipsilaterally at 5 days (mean +/- SEM: 30.5 +/- 5.6) and were maintained until 8 weeks (33 +/- 10.6) after the denervation. In the trigeminal motor nucleus, NADPH-d-positive neurons appeared transiently and bilaterally, peaking at 1 week (663.5 +/- 156.2, ipsilateral side; 687.5 +/- 118.6, contralateral side) after unilateral denervation of the masseteric nerve. In both Vo/Sn and Vc, the number of NADPH-d-positive neurons in the control animals showed a decrease at 3 days but significantly increased from 5 days to 1 week and gradually fell to the control values by 8 weeks after the denervation. There were no significant differences observed between the two sides in either Vo/Sn or Vc. nNOS-positive neurons were similarly distributed and the numbers of labeled neurons were similar to those of NADPH-d-positive neurons after the denervation, although the changes were delayed by approximately 1 week. In conclusion, after unilateral nerve transection, the peak NADPH-d activity occurs 1 week prior to nNOS activity.

Expression of mRNA for plasminogen activators and protease nexin-1 in innervated and denervated mouse skeletal muscle

Plasminogen activators (urokinase-type, u-PA and tissue-type, t-PA) are serine proteases that have been suggested to play important roles in synaptic remodeling. The enzymatic activity of u-PA in particular has previously been shown to increase dramatically after denervation of skeletal muscle. Using (32)P-labeled riboprobes and Northern blots the expression of mRNA for u-PA, t-PA and the inhibitor protease nexin-1 (PN-1) has been studied in innervated and 1-10-days denervated hind-limb muscle from mouse. Using RNA extracted from innervated and 6-days-denervated mouse hemidiaphragm muscles the expression of these mRNAs has also been investigated in synaptic and extrasynaptic muscle regions. For both u-PA and t-PA the observed autoradiographic signals were similar for RNA extracted from innervated and denervated leg muscles. The signals were also similar for RNA extracted from perisynaptic and extrasynaptic regions of hemidiaphragm muscle but u-PA signals were lower in denervated than in innervated hemidiaphragm. No such difference was observed for t-PA. PN-1 mRNA levels were also found to decrease after denervation in the hemidiaphragm but no substantial decrease was observed in denervated hind-limb muscles. No difference was observed between PN-1 expression in perisynaptic and extrasynaptic regions. The effect of denervation on PA enzymatic activity in skeletal muscle is therefore likely to be mediated at some post-transcriptional level.

A novel role for protein tyrosine phosphatase shp1 in controlling glial activation in the normal and injured nervous system

Tyrosine phosphorylation regulated by protein tyrosine kinases and phosphatases plays an important role in the activation of glial cells. Here we examined the expression of intracellular protein tyrosine phosphatase SHP1 in the normal and injured adult rat and mouse CNS. Our study showed that in the intact CNS, SHP1 was expressed in astrocytes as well as in pyramidal cells in hippocampus and cortex. Axotomy of peripheral nerves and direct cortical lesion led to a massive upregulation of SHP1 in activated microglia and astrocytes, whereas the neuronal expression of SHP1 was not affected. In vitro experiments revealed that in astrocytes, SHP1 associates with epidermal growth factor (EGF)-receptor, whereas in microglia, SHP1 associates with colony-stimulating factor (CSF)-1-receptor. In postnatal and adult moth-eaten viable (me(v)/me(v)) mice, which are characterized by reduced SHP1 activity, a strong increase in reactive astrocytes, defined by GFAP immunoreactivity, was observed throughout the intact CNS, whereas neither the morphology nor the number of microglial cells appeared modified. Absence of (3)[H]-thymidine-labeled nuclei indicated that astrocytic proliferation does not occur. In response to injury, cell number as well as proliferation of microglia were reduced in me(v)/me(v) mice, whereas the posttraumatic astrocytic reaction did not differ from wild-type littermates. The majority of activated microglia in mutant mice showed rounded and ameboid morphology. However, the regeneration rate after facial nerve injury in me(v)/me(v) mice was similar to that in wild-type littermates. These results emphasize that SHP1 as a part of different signaling pathways plays an important role in the global regulation of astrocytic and microglial activation in the normal and injured CNS.